Many electronic circuit applications, especially at the integrated circuit level need a stable bias voltage or current for startup. For example, a clock needs a stable bias current and so does a circuit that generates a reference band gap voltage. Further, there is a need, especially at the chip level, to produce a stable bias voltage in a manner that does not take a lot of space on a wafer, does not require a clock on startup, and is stable across the operating temperature range of the device.
One approach to meeting these needs is to utilize Bipolar Junction Transistors (BJTs) that have a VBE relationship wherein the difference between the VBE of two different sized BJTs generates a current that is Proportional To Absolute Temperature (IPTAT), or in other words increases with increasing temperature. Another portion of this circuit can be utilized to generate a current that is Complementary To Absolute Temperature (ICTAT), or in other words decreases with increasing temperature. These two currents with essentially inverse temperature relationships can then be summed with each other to create a current that has a low or zero temperature coefficient. By sending this temperature stable current through a resistor, a voltage can also be obtained. In a perfect world, this would be a temperature stable voltage. However, because resistors have temperature coefficients, the resultant voltage will have a temperature coefficient even though it has been generated from a temperature stable current.
One method to get around this problem is described in a 1999 IEEE article by G. Ripamonti entitled, “Low Power—Low Voltage Band Gap References for Flash-EEPROM Integrated Circuits: Design Alternative and Experiments.” This method suggests generating the two currents described above (IPTAT and ICTAT). Equation 1 in Table 1 describes the creation of IPTAT, while Equation 2 in Table 1 describes the creation of ICTAT. For instance, IPTAT is created by a voltage across RP as shown by Equation 1, while ICTAT is created by a voltage across RC as shown by equation 2. The two currents through the resistors are then summed over a third resistors RBIAS as shown by Equation 3 in Table 1. The Ripamonti method suggests that if all three resistors are constructed of similar material and have similar characteristics, then the resulting voltage (VBIAS) across RBIAS should be temperature independent. This is because the temperature dependencies due to the temperature coefficients of the resistors will cancel each other out and be removed from the ensuing voltage. Further, it is possible to alter the ratio of the currents (IPTAT and ICTAT) by altering the size of the resistors RP and RC that are utilized to create the currents. This scalability is shown by Equation 4 of Table 1, where “A” represents a scalability factor for RP and “B” represents a scalability factor for RC. This then allows for a fully scalable reference voltage value.
TABLE 1Exemplary Current and Voltage equationsWhere the current proportional to absolute temperature is represented as:       I    PTAT    =            Δ      ⁢                          ⁢              V        gs                    R      P      equation 1 And the current complementary to absolute temperature is represented as:       I    CTAT    =            V      EB              R      C      equation 2 Then, a bias voltage independent of temperature is represented as:       V    BIAS    =            R      s        ⁡          (                                    Δ            ⁢                                                  ⁢                          V              gs                                            R            P                          +                              V            EB                                R            C                              )      equation 3 The ratio of the currents used to used to create VBIAS can be altered asshown:       V    BIAS    =            R      s        ⁡          (                                                  Δ              ⁢                                                          ⁢                              V                gs                                                    R              P                                ⁢          A                +                              V            EB                                R            C                              )      equation 4
FIG. 1 is prior art and shows Ripamonti's proposed circuit for summing two currents that have opposing temperature coefficients (one proportional and one complementary) across a resistor to achieve a temperature stable bias voltage. One stage of this circuit generates a current proportional to absolute temperature (IPTAT), another stage generates a current complementary to absolute temperature (IVBE), and a third stage generates a bias voltage by summing the currents on a resistor. In most instances, in the portion of this circuit that is used for generating a current complementary to absolute temperature, the emitter current (IVBE) of bipolar junction transistor Q1 is traditionally so large in comparison to the base current (Ib) of Q1, that the base current is considered negligible. In some instances, where it is difficult to achieve a negligible base current, Ripamonti recommends that careful selection of transistor Q1 and other components can take place during design, so that the base current is negligible at temperatures in the operating range of the device, thus resolving the problem.
However, in situations with very broad operating temperature ranges and/or severe design limitations, it is not practical or possible to design a circuit with a base current that is negligible in comparison to the emitter current that is generated on Q1. One example of a severe design limitation occurs when working with parasitic materials that only allow creation of BJT transistors with extremely low betas, such as a beta of approximately 2 or less. In other instances, when a very high accuracy is needed to create a highly accurate reference current or voltage, it is impossible that any base current could be considered negligible.